Cooling apparatus with dynamic load adjustment

ABSTRACT

A cooling apparatus is disclosed, which may include multiple heat producing units. The cooling apparatus may also have a thermal interface material (TIM) to facilitate heat transfer away from the heat producing units. The cooling apparatus may also have multiple heat sink columns located above, and designed to conduct heat away from, corresponding heat producing units, through thermally conductive contact with corresponding portions of the TIM layer. The cooling apparatus may also have a load plate located above the heat sink columns, designed to hold the heat sink columns in a relatively fixed position above the heat producing units. The TIM layer may have an initial compressed state between the heat sink columns and the corresponding heat producing units. Each of the heat sink columns may be designed so that, in operation, the corresponding portion of the TIM layer may have a further compressed state.

TECHNICAL FIELD

The present disclosure relates to cooling of electronic components. Inparticular, this disclosure relates to a heat removal device withdynamically adjustable thermal characteristics.

BACKGROUND

A heat sink may be used in computers and electronic systems as a passiveheat exchanger, and may act as a reservoir that can absorb an arbitraryamount of heat without significantly changing temperature. Heat sinksmay be used in computers, for example, to cool devices such as centralprocessing units (CPUs) and/or graphics processing units (GPUs).

A heat sink may also dissipate heat produced by electronic devices intoa medium, such as air, water, or a coolant/refrigerant. Heat sinks mayreach a temperature greater than a cooling medium, in order to transferheat across a thermal gradient from an electronic device to the medium,by convection, radiation, or conduction.

A thermal interface material (TIM) may be used to enhance heat transferbetween an electronic device, such as an integrated circuit (IC), and aheat sink, and may be fabricated from thermally conductive material. ATIM may increase thermal conductivity by replacing irregularities andair gaps between adjacent, mating surfaces (e.g., of the IC and the heatsink) with a thermally conductive material.

SUMMARY

Various aspects of the present disclosure may be useful for providingefficient cooling paths for integrated circuits (ICs) and portions ofICs. An adaptive cooling apparatus configured according to embodimentsof the present disclosure may dynamically adjust cooling path length forindividual regions of circuitry, in response to heat produced by theregions.

Embodiments may be directed towards a cooling apparatus. The coolingapparatus may include a plurality of heat producing units (HPUs), eachhaving a top surface. The cooling apparatus may also include a firstthermal interface material (TIM) layer. The first TIM layer may have atop surface, and a bottom surface adjacent to, and in thermallyconductive contact with, the top surfaces of the heat producing units,which may facilitate heat transfer away from the heat producing units.The cooling apparatus may also have a plurality of heat sink columns,each with a top surface, and a bottom surface located above acorresponding heat producing unit that is above and in thermallyconductive contact with, the top surface of a corresponding portion ofthe first TIM layer. Each heat sink column may be designed to conductheat away from the corresponding portion of the first TIM layer. Thecooling apparatus may also include a load plate located above the topsurfaces of the heat sink columns and designed to, in a latched state,hold the top surfaces of the heat sink columns in a relatively fixedposition above the heat producing units. In a latched state, the firstTIM layer may have an initial compressed state between the heat sinkcolumns and the corresponding heat producing units, and each of theplurality of heat sink columns may be designed so that, in operation,the corresponding portion of the first TIM layer has a furthercompressed state.

Embodiments may also be directed towards a method for cooling heatproducing units (HPUs). The method may include positioning a first TIMlayer on a top surface of at least one HPU of a plurality of HPUs, tofacilitate heat transfer away from the at least one HPU. The method mayalso include positioning a plurality of heat sink columns abovecorresponding heat producing units and above the first TIM layer, andpositioning a load plate above the heat sink columns. The method mayinclude exerting a compressive force on each heat sink column, by movingthe load plate into a latched position, and conducting heat away fromthe first TIM layer with at least one of the plurality of heat sinkcolumns. The method may further include adjusting compression of a firstportion the first TIM layer that is above a first heat producing unit bya first amount, and adjusting compression of a second portion of thefirst TIM layer that is above a second heat producing unit by a secondamount that is different than the first amount.

Embodiments may be directed towards a cooling apparatus. The coolingapparatus may include a plurality of heat producing units, each having atop surface, and a first thermal interface material (TIM) layer, havinga top surface, and a bottom surface adjacent to the top surfaces of theheat producing units, and configured to facilitate heat transfer awayfrom the heat producing units. The cooling apparatus may also include aplurality of heat sink columns, each column having a top surface, and abottom surface located above a corresponding heat producing unit andabove the top surface of the first TIM layer. Each of the heat sinkcolumns may be configured to conduct heat away from the first TIM layer,and to increase a respective force on the top surface of thecorresponding heat producing unit as a result of thermal expansion. Thecooling apparatus may also include a load plate, located above the topsurfaces of the heat sink columns. The load plate may be configured to,in a latched state, provide at least a portion of the respective forceon the top surface of each heat sink column, in a direction normal tothe top surface of the column, to compress the first TIM layer betweenthe heat sink column and the corresponding heat producing unit.

Aspects of the various embodiments may be used to enable electroniccircuits to operate stably and reliably within a limited operationaltemperature range. Aspects of the various embodiments may also be usefulfor providing cost-effective cooling apparatuses for use with heatproducing electronic devices, by using existing and proven heat sink,TIM and IC technologies.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings included in the present application are incorporated into,and form part of, the specification. They illustrate embodiments of thepresent disclosure and, along with the description, serve to explain theprinciples of the disclosure. The drawings are only illustrative ofembodiments of the disclosure and are not limiting.

FIG. 1 is a front view of an adaptive cooling apparatus including a loadplate, heat sink columns, thermal interface material (TIM) layer, heatproducing units (HPUs) and a substrate, according to embodiments of thepresent disclosure.

FIG. 2 is a top view of an adaptive cooling apparatus including a loadplate, heat sink columns, HPUs and a substrate, according toembodiments.

FIG. 3 is a front view of an adaptive cooling apparatus including twothermal interface material (TIM) layers, a lid, heat producing units anda substrate, according to embodiments.

FIG. 4 is a front view of an adaptive cooling apparatus including a heatpipe, two thermal interface material (TIM) layers, and a lid, accordingto embodiments.

FIG. 5 is a cross-sectional view of a heat pipe attached to a heat sinkcolumn, according to embodiments.

FIG. 6 includes 3 front views of a heat sink column, and a thermalgradient corresponding to an expanded heat sink column, according toembodiments.

FIG. 7 is a flow diagram illustrating a method for cooling heatproducing units, according to embodiments.

In the drawings and the Detailed Description, like numbers generallyrefer to like components, parts, steps, and processes

DETAILED DESCRIPTION

Certain embodiments of the present disclosure can be appreciated in thecontext of increasing the adaptability and efficiency of cooling systemsfor ICs, or regions of an IC, that may be used in a computer orelectronic system. Such IC types may include, but are not limited to,central processing units (CPUs), graphics processing units (GPUs), andinterface chips. While not necessarily limited thereto, embodimentsdiscussed in this context can facilitate an understanding of variousaspects of the disclosure. Certain embodiments may also be directedtowards other equipment and associated applications, such as increasingcooling system adaptability and efficiency for high-power semiconductordevices such as power transistors and amplifiers, which may be used in awide variety of electronic systems. Such ICs may include, but are notlimited to, analog circuits fabricated in various semiconductortechnologies. Embodiments may also be directed towards cooling systemsfor other types of electronic devices including, but not limited to,laser diodes and light emitting diodes (LEDs). The intrinsic heatdissipation capability of these types of devices may be insufficient tomoderate the device's temperature, without the use of an additionalcooling device.

Various embodiments of the present disclosure relate to adaptive coolingapparatuses configured to remove heat from ICs by thermal conduction,which can be useful for providing robust protection of IC's from damageresulting from high operating temperatures. Stable and reliableperformance of an IC may result from the use of an adaptive coolingapparatus. The adaptive cooling apparatus may protect a number of ICs(which may each dissipate a different amount of power), and may beconfigured to provide independently adapted, efficient cooling paths foreach IC. Efficient heat transfer paths provided by the adaptive coolingapparatus may allow ICs to operate safely and reliably at high operatingfrequencies and workloads.

An adaptive cooling apparatus designed according to certain embodimentsmay be compatible with existing and proven IC technologies andelectronic systems, and may be a useful and cost-effective way toprotect ICs from permanent damage. An adaptive cooling apparatusconstructed according to embodiments of the present disclosure mayprotect an IC from high operating temperatures, by responding to heatdissipated by the IC.

Continuing trends in electronic packaging involves increasing electroniccircuit density through the placement of various functional units inclose proximity to each other, which may provide distinct performance,density and cost advantages over less integrated solutions. Thefunctional units may have a wide variation in the amount of heat theydissipate during operation, resulting both from the type/characteristicsof the circuits included in the units, and from the type of workload(throughput) the unit may be executing at a particular time. Anintegrated electronic device, such as a highly integrated chip (IC), ormulti-chip module, may therefore contain a number of regions ofcircuitry, located adjacent to each other, each having differing powerdissipation characteristics from neighboring units.

A heat sink may be used to transfer heat from the chip or module to amedium into which it may be dissipated, for example, air, water, or arefrigerant. A thermal interface material (TIM) may be used to provide ahighly conductive thermal path between the heat producing chip or moduleand the heat sink.

A thermal interface material (TIM) may facilitate heat transfer byfilling surface irregularities and air gaps between two mating,thermally conductive surfaces in close proximity with a material havinga higher thermal conductivity than air. The thermal conductivity of aTIM layer may be less than the thermal conductivity of a heat sinkmaterial, so a corresponding design objective may be to maximize heattransfer by reducing the thickness of the TIM layer to a minimumthickness needed to fill in surface irregularities and air gaps. Thethickness of a compressible TIM layer may be reduced by the applicationof pressure on it surfaces.

A heat sink composed of a single mass of thermally conductive material,for example, a copper or aluminum block, having a surface profiledesigned to match the profile of one or more heat producing IC's, orsections of an IC, may be used as a heat removal device. A TIM layer maybe placed between the IC(s) and the heat sink to enhance thermalconductivity. When one or more ICs (e.g., of a multichip module) are inoperation, they may each dissipate varying amounts of heat. For example,a processor chip may dissipate over 100 W, in a highly active state, anda less active device, such as an infrequently accessed memory chip, maydissipate much less power, for example 2W. The disparity in the amountof heat dissipated by these possibly adjacent heat producing units(HPUs) may result in thermal expansion of their respective chips, and/orlocal areas of the heat sink. A local thermal expansion may have theeffects of changing the compression of a region of the TIM layer, andpossibly displacing the heat sink from the chip or multichip module. Anoverall displacement of the heat sink may result in increased distancebetween particular regions (HPUs) of the die or multi-chip module, andcorresponding mating areas of the heat sink. This increase in distancemay increase the thermal resistance and thermal path length across theTIM layer, resulting in a reduction in cooling efficiency for certainheat producing units.

Certain embodiments relate to the dynamic adjustment of compressiveforces exerted on regions of a TIM, and respective heat producing units(HPUs), by corresponding individual heat sink columns, in response toindividual HPU temperature fluctuations. FIG. 1 is a front view of anadaptive cooling apparatus 100, generally used for cooling HPUs 124,according to embodiments of the present disclosure. An adaptive coolingapparatus 100 may include a number of HPUs 124, which may be arrangedand/or mounted on a substrate 118. According to certain embodiments, anHPU 124 may be an IC or a region of an IC, and the substrate 118 may bea planar structure used for mechanical mounting and electricalinterconnection of ICs, consistent with electronic packaging used incomputer and other electronic systems. For example, in certainembodiments, HPUs 124 may be processor chips (ICs), and the substrate118 may be an organic laminate chip carrier, used to electricallyinterconnect the processor chips. Adaptive cooling apparatus 100 maydynamically alter its thermal characteristics in response to the amountof heat dissipated by each HPU 124, and may be particularly useful inensuring reliable operation of HPUs 124.

A bottom surface of the (first) TIM 120 may be located on top of, and inthermally conductive contact with the top surface of HPUs 124. Thermallyconductive contact (of two mating surfaces) may enable the transfer ofheat from one surface to the other, and may require a specified level ofpressure on the surfaces.

TIM 120 may be useful for conducting heat from the HPUs 124 to the heatsink columns 106. The bottom surface of each heat sink column 106 maycorrespond to a particular HPU 124, and be in thermally conductivecontact with the top surface of a corresponding portion of the (first)TIM 120 layer. Heat sink columns 106 may be useful in conducting heataway from, and limiting the operating temperature of HPU 124.

According to embodiments, the load plate 104 may be attached to frame122, and may (in a latched state) provide a compressive force onto thetop surface of the heat sink columns 106. The compressive force may betransferred by heat sink column 106 to TIM 120 and HPUs 124, and used toreduce TIM 120 thickness, relative to an uncompressed TIM 120 thickness.Latch 102 may be useful to secure the load plate 104 in a latched(closed) position. FIG. 1 depicts load plate 104 in both open andlatched state (closed) positions.

HPUs 124 may include certain regions of an IC (chip) having heatproducing characteristics that may differ from the heat producingcharacteristics of other, possibly adjacent, regions. An HPU 124 mayalso be an entire chip, mounted on a substrate, which has heat producingcharacteristics that may differ from the heat producing characteristicsof another chip mounted on the same substrate. Heat producingcharacteristics may include minimum and maximum HPU power dissipationlevels (i.e., wattage). HPU 124 power dissipation may vary based uponboth the type of circuits within the HPU 124, and variation inworkloads, or activity level of the HPU 124.

As an example, an HPU 124 may be a processor chip which may dissipatemore than 100 W, when running a certain (high utilization) workload, andmay consume less than 10 W while running in an “idle” (low utilization)state. Another type of HPU 124, for example, may be a dynamicrandom-access memory (DRAM) chip, which may dissipate less than 8 W whenit is heavily accessed and approximately 1 W when it is not beingaccessed. An HPU may also be a certain region of circuitry on an IC,such as a processor core, memory unit, or RF transmitter circuit.

The (first) TIM layer 120 may have an initial compressed state heightbetween the heat sink columns 106, and the corresponding heat producingunits 124. The initial compressed height of TIM 120 may result from thecompressive force applied by the load plate 104, in a direction normalto the top of the heat sink column(s) 106, and transferred to the TIMlayer 120 by corresponding heat sink column(s) 106. This initialcompressed state height may be useful in producing a limited thermalpath length (through the TIM 120 layer), and limiting the thermalresistance of the TIM layer 120, which may enable efficient heattransfer between an HPU 124 and a heat sink column 106.

In certain embodiments, thermal expansion of one or more of the heatsink columns 106 may result in increased compressive force on the topsurface of the corresponding portions of the TIM 120, and on the heatproducing unit(s) 124. The increase in force on the TIM 120 may cause itto further compress, which may result in a decrease of thermal pathlength and thermal resistance (relative to the initial compressedstate), and a corresponding increase in heat transfer efficiency betweenan HPU 124 and a heat sink column 106.

According to embodiments, a heat sink column 106 may be fabricated froma number of thermally conductive materials, including, but not limitedto, aluminum, aluminum alloys, copper, graphite, and carbon nanotubecomposites. Each of the heat sink columns 106 may be designed so that,during operation of the HPUs 124, the thermal expansion of the heat sinkcolumn 106 (in response to heat received from a corresponding HPU 124)compresses a corresponding portion of the first TIM 120 layer, to afurther compressed state, which may be thinner than the initialcompressed state (before heat sink column 106 thermal expansion) of thefirst TIM 120 layer.

Each heat sink column 106 may have a certain coefficient of thermalexpansion (CTE), derived from characteristics of a material or set ofmaterials it is fabricated from. Individual heat sink columns may befabricated from different materials and/or sets of materials, and thusmay be designed to have different CTEs. Heat sink columns may also bedesigned with differing initial (unexpanded) heights, i.e., the distancebetween the top surface and the bottom surface of a heat sink column106. Individual heat sink column 106 CTEs may be useful in adjusting theresulting compression of a portion of the TIM 120 layer corresponding toa certain HPU 124. Individual heat sink column 106 initial heights maybe useful in compensating for variations among of the thicknesses ofcertain HPU 124 chips, and/or thickness of HPU 124 chip interconnect(e.g., solder balls) to a substrate 118.

According to certain embodiments, the design of adaptive coolingapparatus 100 may enable each heat sink column 106 to expand andcontract independently from other heat sink columns 106, in response tothe individual levels of heat produced by their corresponding HPUs 124.The ability of heat sink columns 106 to independently respond to theirrespective HPUs 124 may be useful in preventing the thermal expansion ofone HPU 124, or of a certain heat sink column 106, from causingincreased separation between another HPU 124 and its respective heatsink column 106. Increased distance between an HPU 124 and a heat sinkcolumn 106 may cause an increased TIM 120 thickness, and may reduce heattransfer efficiency for a particular HPU 124, according to embodiments.

A heat sink column 106 may dissipate heat through the use of coolingfins (for air cooling), embedded or attached fluid channels (for wateror refrigerant-based cooling), or attachment to a heat pipe (forphase-change cooling). Heat dissipation from the heat sink columns 106may create a thermal gradient across the heat sink column 106, which mayallow continued heat removal from HPU(s) 124.

The load plate 104 may be located above the top surfaces of the heatsink columns 106, and maybe designed to (in a latched state) maintainthe top surfaces of the heat sink columns 106 in a relatively fixed(coplanar) position above the HPUs 124. The connection of the load plate104 to the (rigid) frame 122, which may also extend below substrate 118,may enable this relatively fixed positioning of the top surfaces of theheat sink columns 106 relative to the substrate 118. The frame mayinclude a backing (stiffening) plate located below the substrate 118,which may used to support it.

FIG. 2 is a top view of an adaptive cooling apparatus 200, consistentwith FIG. 1, and including a load plate 104, heat sink columns 106, heatproducing units 124, frame 122, latches 102, and a substrate 118,according to embodiments. The bottom surface of heat sink columns 106may be designed to overlap the top surfaces of HPUs 124, which may beuseful in utilizing the entire area of the top surfaces of the HPUs forheat transfer. FIG. 2 illustrates HPUs 124 arranged in a 2-dimensionalarray, according to embodiments. Certain embodiments may include HPUs124 arranged in a 1-dimensional array. Embodiments may include HPUs 124and corresponding heat sink columns 106 having various sizes and shapes.Embodiments may include guides or other hardware elements designed tomaintain appropriate alignment of heat sink columns 106 relative to HPUs124.

FIG. 3 is a front view of an adaptive cooling apparatus 300, similar toFIG. 1, including (first) TIM layer 320, (second) TIM layer 314, a lid316, heat sink columns 306, heat producing units 324, and a substrate318, according to embodiments. The lid 316 may be located on, and inthermally conductive contact with, the top surface of the (first) TIMlayer 320. Second TIM layer 314 may be located between, and in thermallyconductive contact with the lid 316 and the bottom surface of the heatsink columns 306. The lid 316 may be useful for spreading heatdissipated by one or more of the heat producing units 324 to a largersurface area, where it may be more effectively removed. Lid 316 may alsobe useful in providing one or more of the HPUs 324 with shielding fromESD events, and provide mechanical stability for the assembly ofsubstrate 318, HPUs 324 and lid 316. In certain embodiments, a lid 316may consist of a planar section of thermally conductive material, suchas copper or aluminum. In certain embodiments, the lid 316 may includesides used to further encompass HPUs 324. In certain embodiments, thelid 316 may cover or encompass one or more than one HPU 324.

View 300 depicts heat sink columns 306 positioned in a latched state,which may cause a compressive force to be exerted on both TIM 314 andTIM 320, which may result in the compression of both TIMs, relative toan uncompressed state of the TIMs 314, 320. The amount of compression ofeach TIM 314 and TIM 320 may depend on the type of material used foreach TIM, according to embodiments.

FIG. 4 is a front view of an adaptive cooling apparatus 400, consistentwith FIG. 1, and including a heat pipe 412, heat sink columns 406 twothermal interface material (TIM) layers 414, 420, and a lid 416,according to certain embodiments. A heat pipe 412 may be attached to,and in thermally conductive contact with, at least one heat sink column406. Heat pipe 412 may be useful for to equalize temperature (transferheat) from one sink column 406 to another or to remove heat from aparticular heat sink column 406, corresponding to a particular HPU 424.

A heat pipe 412 may be thermally connected to a heat sink column 406 bya soldered connection, consistent with connections used in electronicsand mechanical assembly. A soldered connection may create a more robustand efficient thermal path between the heat pipe 412 and a heat sinkcolumn 406 than may be created with other materials (e.g., a TIM layer).The heat pipe 412 may also be useful in transferring heat from HPUs 424by coupling heat sink columns to other dissipative structures or devicessuch as heat exchangers and/or fin assemblies. A heat pipe 412 may beconsistent with phase-change heat pipes used in computer, electronic andother industries, and may include a coolant contained in a hollow,sealed, thermally conductive structure.

FIG. 5 depicts a cross-sectional view 500 of a heat pipe 512 attached toa heat sink column 506 by a solder joint 513, according to embodiments.The solder joint 513 may provide a thermally conductive path throughwhich heat may flow between the heat sink column 506 and the heat pipe512. In embodiments, the solder joint 513 may be created using soldercompounds and fluxes consistent and compatible with those used in theelectronics and mechanical assembly industries, such as various tin/leadand lead-free alloys. A solder joint is depicted between one pair ofmating surfaces of heat sink 506 and heat pipe 517, however embodimentsmay include joints between multiple pairs of mating surfaces.

FIG. 6 includes 3 front views (630, 635 and 640) of an arrangement of aheat sink column 606, a TIM 620, and a HPU 624, according to embodimentsconsistent with FIG. 1. FIG. 6 also includes a depiction of a thermalgradient (view 645), corresponding to a heat sink column in an expandedstate (606, view 640), according to embodiments.

The depiction of TIM 620, located between HPU 624 and heat sink column606, in view 630, illustrates an uncompressed state of the TIM 620,corresponding to a lack of compressive force from a load plate (e.g.,104, FIG. 1), according to embodiments. This may result from the loadplate (104, FIG. 1) either being in an open position (FIG. 1), or beingdetached from the frame (122, FIG. 1). In this state, the temperature ofheat sink column 606 may be equal to an ambient temperature, T_(AMB)626, such as room temperature, and may have an (unexpanded) initialheight, H_(I), and the TIM 620 may have an uncompressed thickness,T_(I).

Initial height H_(I), of heat sink column 606, may be based on a thermalmass used to cool HPU 624, a specified amount of thermal expansion ofthe heat sink column 606, cooling apparatus cost and size limitations,and other design constraints, according to embodiments. As an example,H_(I) may be specified to be 3 cm (30,000 μm). An uncompressedthickness, T_(I), of TIM 620, may generally be specified to be verysmall, compared to H_(I). A T_(I) for an indium layer, for example, maybe 0.25 mm (250 μm). The value of T_(I) may vary based upon the type ofTIM layer 620, and related design constraints, such as a specifiedthermal resistance.

View 635 depicts an initial compressed state of TIM 620, resulting fromthe application of a compressive force on the top of heat sink column606, by load plate 604, according to embodiments. This compressive forcemay result from load plate 604 being moved into a latched position (see104, FIG. 1). The compressive force may result in heat sink column 606being displaced downwards by a distance, C, from its initial position(view 630). Correspondingly, the thickness of TIM 620 may be reduced bya compression distance of C, to a compressed thickness, T_(C). Thereduction in thickness (thermal path length) of the TIM 620 (from T_(I)to T_(C)) between the HPU 624 and the heat sink column 606, may resultin lower thermal resistance, and the more efficient transfer of heatfrom the HPU 624 to the heat sink column 606. In the initial compressed(view 635), heat sink column 606 may be maintained at the ambienttemperature, and (unexpanded) initial height, H_(I).

As an example, the value of C may be 100 μm. A compressed thickness,T_(C), may therefore be calculated as:

T _(C) =T _(I) −C, or

250 μm−100 μm=150 μm (0.150 mm).

View 640 depicts a further compressed state of TIM 620. In this state,HPU 624 may generate heat that may be subsequently transferred to heatsink column 606, causing it to thermally expand, by a change in length(E), to an expanded height H_(E), which may be greater than its initialheight, H_(I).

The change of height (E) of the heat sink column 606, due to thermalexpansion, may be expressed by the (simplified) equation:

E=H _(I) ×CTE×ΔT

Where:

-   -   H_(I)=Initial height of heat sink column 606    -   CTE=the column's 606 coefficient of thermal expansion, which may        be expressed in units of parts-per-million per degree Celsius        (PPM/° C.).    -   ΔT=the (average) change in temperature of the heat sink column        606

As an example, if the initial height, H_(I), of heat sink column 606 is3 cm (30,000 μm), and the heat sink column 606 is fabricated fromaluminum, having a CTE of 22.2 (PPM/° C.), and an average ΔT (over theheight of the heat sink column 606) is 25° C., then the change of height(E) may be calculated as:

E=30,000 μm×22.2 PPM/° C.×25° C.=16.65 μm (11% of the compressedthickness of 150 um)

An expanded height, H_(E), of heat sink column 606, H_(E), may becalculated as:

H _(E) =H _(I) +E, or 30,000 μm+17 μm=30,017 μm (30.017 mm).

The expansion of heat sink column 606 may result in an increase incompressive force applied to the TIM 620, which may further compress it,and reduce its thickness by an amount (E), from T_(C) to T_(E). Thecompressive force applied to TIM 620 in view 640 may be the sum of theinitial compressive force from load plate 604, and the additionalcompressive force, resulting from the thermal expansion of heat sinkcolumn 606.

A further compressed thickness, T_(E), may therefore be calculated as:

T _(E) =T _(C) −E, or 150 μm−17 μm=133 μm (0.133 mm).

The further reduction in thickness of the TIM 620, (from T_(C) toT_(E)), may result in a shorter thermal path, T_(E), between the HPU 624and the heat sink column 606, which may result in a further lowering ofthermal resistance, and more efficient heat transfer to the heat sinkcolumn 606. In certain embodiments, the change in length (E), due tothermal expansion of heat sink column 606 may be as high as 30% of acompressed thickness, T_(C), of TIM 620. In some embodiments, E may beas low as 10% of T_(C).

In embodiments, a heat sink column 606 may expand or contract inresponse to an increase or decrease, respectively, of its temperature,which may result from heat transferred from HPU 624. This expansion andcontraction may be useful in varying the thickness of TIM 620, and thusproviding a shorter thermal path and more efficient cooling of HPU 624,during periods of high heat dissipation from HPU 624. More efficientcooling of HPU 624 may result in a lower chip operating temperature, andmore reliable operation.

View 645 depicts a thermal gradient corresponding to view 640 of heatsink column 606, which may have a bottom surface coupled to, andreceiving heat from, heat producing unit 624, through TIM 620. Theoperating temperature of the bottom surface of heat sink column 606 isdepicted as T_(OPER) 628. The top surface of heat sink column 606 mayhave a temperature T_(AMB) 626, less than the operating temperatureT_(OPER) 628. T_(AMB) 626 may be an ambient temperature of a coolingmedium (such as surrounding air), or may be a temperature above thetemperature of the medium. The X-axis of view 645 corresponds to therelative temperature, between T_(AMB) 626 and T_(OPER) 628, of locationsalong the height of the heat sink column 606 (view 640). The Y-axiscorresponds to a location on the heat sink column 606 relative to HPU624. The line 650 may indicate the temperature gradient between the ends(top and bottom surfaces) of heat sink column 606. The temperaturegradient 650 may be linear in some embodiments, and may not be linear incertain embodiments. Embodiments that include a heat pipe (412, FIG. 4)may have a temperature gradient differing from 650, resulting from theeffectiveness of the heat pipe as a heat transfer device.

Specific numerical dimensions, temperatures, temperature gradients andCTEs discussed herein are included as examples, for the illustration ofoperating principles of the present disclosure, and do not limit thedisclosure in any way. A variety of materials, with associated materialproperties, such as CTE and thermal conductivity, may be used in thepractice of the present disclosure.

FIG. 7 is a flow diagram illustrating a method for cooling heatproducing units, according to embodiments. The process 700 moves fromstart 702 to operation 704. Operation 704 generally refers topositioning a TIM layer on top of at least one of a plurality of HPUs,as depicted, for example by 120 (FIG. 1). The TIM layer may be, forexample, a carbon fiber pad or a thermal grease/paste. The TIM layer maybe positioned to completely cover the top surface of one or more HPUs,in order to enable efficient heat transfer from the HPU to acorresponding heat sink column. A TIM layer or material may be chosen tohave a specified thickness, compressibility, and thermal conductivity,to be suitable for a particular application. Alignment holes or guidesmay be used in conjunction with a TIM layer, for example, a carbon fiberpad, in order to facilitate positioning it on top of one or more HPUs. ATIM material that is a thermal grease/paste may be applied, at acontrolled thickness, to the HPU(s) by the use of a spreader or asqueegee. Once the TIM layer is positioned on top of at least one HPUthe process moves to operation 706.

Operation 706 generally refers to positioning one or more heat sinkcolumns on top of the TIM layer and one or more HPU, as depicted, forexample by 106 (FIG. 1). A heat sink column may be manufactured with aplanar, smooth bottom surface, which may limit surface irregularities,air gaps, and thermal resistance between the heat sink column and amating surface of a corresponding HPU. The bottom surface of the heatsink column may also be designed to overlap and completely cover thecorresponding top surface of the HPU, in order to efficiently use thetop surface of the HPU for heat transfer. A frame (e.g. 122, FIG. 1) mayinclude members used to align heat sink columns such as 106 (FIG. 1)with HPUs such as 124 (FIG. 1). Once one or more heat sink columns arepositioned on top of the TIM layer and one or more HPU, the processmoves to operation 708.

Operation 708 generally refers to positioning a load plate on the topsurface of one or more heat sink columns. The load plate may be attachedto a frame (e.g., 122, FIG. 1) by a hinge, clips, or a latch (e.g., 102,FIG. 1), and may have a bottom planar surface designed to contact, andexert a force on, a top planar surface of one or more heat sink columns.Once the load plate is positioned on the top surface of one or more heatsink columns, the process moves to operation 710.

Operation 710 generally refers to exerting a compressive force on one ormore heat sink columns by placing the load plate in a latched position.The height of the heat sink columns may be designed, in conjunction witha selected TIM layer, to transfer a compressive force from the loadplate, in a latched position, to the TIM layer. The heat sink columnsmay have a variety of heights designed to be compatible with ranges ofHPU heights and TIM thicknesses. The latch (e.g., 102, FIG. 1) may beused to secure the load plate (e.g., 104, FIG. 1) to the frame (e.g.,122, FIG. 1) once the load plate is moved into the latched position.Once a compressive force is exerted on one or more heat sink columns theprocess moves to operation 712.

Operation 712 generally refers to conducting heat away from the TIMlayer (and corresponding HPU) by a heat sink column. When the heat sinkcolumn(s), TIM layer, and corresponding HPU(s) are compressed togetherby the force exerted by the load plate, and an HPU dissipates heat, theheat may be conducted from the HPU, through the TIM, and into thecorresponding heat sink column. Once heat is conducted away from the TIMlayer, the process moves to operation 714.

Operation 714 generally refers to adjusting compression of a portion ofa TIM layer corresponding to a (first) HPU. As the (first) HPUdissipates heat, the corresponding, thermally connected heat sink columnmay increase in temperature, causing it to expand. This expansionagainst the load plate may increase the compressive force exerted on thecorresponding TIM layer, causing a reduction in the thermal path lengthbetween the HPU and the heat sink column. This reduction and thermalpath length may result in more efficient heat transfer between the HPUand the heat sink column. Once the (first) HPU TIM layer compression isadjusted the process moves to operation 716.

Operation 716 generally refers to adjusting compression of a portion ofa TIM layer corresponding to a (second) HPU. As the (second) HPUdissipates heat, the corresponding, thermally connected, heat sinkcolumn may increase in temperature, causing it to expand. This expansionagainst the load plate may increase the compressive force exerted on thecorresponding TIM layer, causing a reduction in the thermal path lengthbetween the HPU and the heat sink column. This reduction and thermalpath length may result in more efficient heat transfer between the HPUand the heat sink column. The amount of expansion of the second HPU, andcompression of the corresponding portion of the TIM layer may bedifferent than, and independent of, the amount of expansion of the firstHPU, as a result of possibly differing amounts of heat dissipated by thefirst and second HPUs. The expansion of heat sink columns in operations714 and 716 may be proportional to a thermal gradient across the heightof each respective heat sink column. Once the (second) HPU TIM layercompression is adjusted the process 700 may end at block 718.

Although the present disclosure has been described in terms of specificembodiments, it is anticipated that alterations and modificationsthereof may become apparent to those skilled in the art. Therefore, itis intended that the following claims be interpreted as covering allsuch alterations and modifications as fall within the true spirit andscope of the disclosure.

What is claimed is:
 1. A method for cooling heat producing units,comprising: using a device having: a first thermal interface material(TIM) layer on a top surface of at least one heat producing unit of aplurality of heat producing units, to facilitate heat transfer away fromthe at least one heat producing unit; a plurality of heat sink columnsabove corresponding heat producing units and above the first TIM layer;and a load plate above the heat sink columns, by: exerting a compressiveforce on each heat sink column, by moving the load plate into a latchedposition; conducting heat away from the first TIM layer with at leastone of the plurality of heat sink columns; adjusting, in response to afirst level of heat produced by a first heat producing unit, compressionof a first portion the first TIM layer that is above the first heatproducing unit by a first amount; and adjusting, in response to a secondlevel of heat produced by a second heat producing unit, compression of asecond portion of the first TIM layer that is above a second heatproducing unit by a second amount that is different than the firstamount.